DLC coating for earth-boring bit bearings

ABSTRACT

An earth-boring bit has a bearing member having a DLC coating. The bearing member locates between a bearing pin and a cone of the bit. The bearing member may be a thrust washer or a bearing sleeve. The DLC coating is diamond-like carbon that may be coated by different processes onto bearing member.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This invention is a continuation-in-part of application Ser. No.10/223,533, filed Aug. 19, 2002.

FIELD OF THE INVENTION

[0002] This invention relates in general to earth-boring bits,especially the bearings for earth-boring bits of the rolling conevariety. More particularly, the invention relates to coatings on thebearings for enhancing wear resistance.

BACKGROUND INFORMATION

[0003] In drilling boreholes in earthen formations by the rotary method,earth-boring bits typically employ at least one rolling cone cutter,rotatably mounted thereon. The bit is secured to the lower end of adrillstring that is rotated from the surface or by downhole motors. Thecutters mounted on the bit roll and slide upon the bottom of theborehole as the drillstring is rotated, thereby engaging anddisintegrating the formation material. The rolling cutters are providedwith teeth that are forced to penetrate and gouge the bottom of theborehole by weight from the drillstring.

[0004] As the cutters roll and slide along the bottom of the borehole,the cutters, and the shafts on which they are rotatably mounted, aresubjected to large static loads from the weight on the bit, and largetransient or shock loads encountered as the cutters roll and slide alongthe uneven surface of the bottom of the borehole. Thus, mostearth-boring bits are provided with precision-formed journal bearingsand bearing surfaces, as well as sealed lubrication systems to increasedrilling life of bits. The lubrication systems typically are sealed toavoid lubricant loss and to prevent contamination of the bearings byforeign matter such as abrasive particles encountered in the borehole. Apressure compensator system minimizes pressure differential across theseal so that the lubricant pressure is equal to or slightly greater thanthe hydrostatic pressure in the annular space between the bit and thesidewall of the borehole.

[0005] The bearing surfaces include a thrust shoulder formed on thebearing pin perpendicular to the axis of the bearing pin. A matingthrust shoulder is formed in the cavity of the cone. A partiallycylindrical journal bearing surface is formed around part of the bearingpin for engaging a mating surface in the cavity of the cone. In thepast, inlays of a hard material, such as Stellite, have been placed onthe thrust shoulder and on the journal bearing surface. Also, a hardenedring has been mounted in the cavity of the cone for engaging the inlayon the journal bearing surface.

[0006] Very hard, wear-resistant layers and coatings have been developedfor a variety of purposes, such as those employing diamond. Thesecoatings, however, generally need to be applied at high temperatures andhigh pressures and are applied after the steel member has been hardened.If the high temperatures exceed the lowest transformation temperature ofthe steel member, such as the temperature at which the steel member hasbeen tempered, this would adversely affect the properties of the sealmember.

[0007] U.S. Pat. No. 6,209,185 to Scott discloses applying a diamondlayer to a substrate, then attaching the diamond layer to a rigid sealring. This avoids having to heat the hardened ring beyond its lowesttransformation temperature, but it does require attachment by brazing,epoxy or the like. U.S. Pat. No. 6,045,029 to Scott discloses forming adiamond layer directly on a rigid seal ring by a process that isaccomplished at a temperature lower than the lowest transformationtemperature of the metal of the seal ring. This may be done in anamorphic diamond process or by forming the diamond layer separately andattaching it to the rigid ring of the seal.

SUMMARY OF THE INVENTION

[0008] In this invention, rather than a diamond coating, a diamond-likecoating (DLC) is applied. A DLC coating is a form of meta-stableamorphous carbon or hydrocarbon polymer with properties very similar tothose of diamond. It is a vapor deposited carbon coating with a mixtureof sp3 and sp2 bonds between the carbon atoms and could be doped withone or more alloying elements such as silicon, boron, boron nitride, andone ore more refractory metallic elements, such as tantalum, titanium,tungsten, niobium or zirconium. The designation sp3 refers to thetetrahedral bond of carbon in diamond, while the designation sp2 is thetype of bond in graphite. As DLC has a certain percentage of both, thehardness is less than diamond and between diamond and graphite.

[0009] The DLC coating is applied to the seal face of a bearing memberafter it has been hardened and tempered. It is applied at a temperaturelower than the lowest transformation temperature so as to notdetrimentally affect the dimensions or hardness of the substrate body ofthe thrust member. In one process, it is performed by the decompositionof a carbon and hydrogen compound, such as acetylene, in the presence ofa plasma. The process is carried out until the coating has a thicknessin the range from about 1 to 10 micrometers. The Knoop scale hardness isin the range from 2,000 to 5,000.

[0010] In one embodiment, the bearing member that has the DLC coatingcomprises a thrust washer that locates between the thrust shoulders ofthe bearing pin and the cone. Also, the bearing sleeve that fits in thecone and engages the bearing pin preferably contains a DLC coating on atleast one side. In the first embodiment, the bearing pin thrust shoulderand journal bearing surface have inlays of a hard, wear resistantmaterial such as Stellite. In an alternate embodiment, the DLC coatingis also applied to the bearing pin thrust shoulder and journal bearingsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a sectional view of a portion of an earth-boring bitconstructed in accordance with this invention.

[0012]FIG. 2 is a perspective view of a journal bearing sleeve of thebit of FIG. 1.

[0013]FIG. 3 is a perspective view of a thrust washer of the bit of FIG.1.

[0014]FIG. 4 is a schematic sectional view of a portion of the thrustwasher of FIG. 3.

[0015]FIG. 5 is a side view of part of a bearing pin of an alternateembodiment.

[0016]FIG. 6 is a graph illustrating a thrust wear test.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Referring to FIG. 1, bit 11 has at least one bit leg 13 andnormally three. Each bit leg 13 has a bearing pin 15 that extendsdownward and inward toward an axis of rotation of bit 11. Bearing pin 15has a cylindrical nose 17 on an inner end that is of lesser diameterthan remaining portions of bearing pin 15. An inward facing annularthrust shoulder 19 surrounds nose 17. Thrust shoulder 19 is located in aplane perpendicular to an axis of bearing pin 15. In this embodiment,thrust shoulder 19 optionally has an inlay 21 of a hard, wear resistantmaterial, such as Stellite. Similarly nose 17 may have an inlay 23 ofthe same wear resistant material on its cylindrical exterior.

[0018] Bearing pin 15 has a partially cylindrical journal bearingsurface 25 that extends around its lower side. In this embodiment, anoptional inlay 27 of a hard wear resistant material, such as Stellite,is located in journal bearing surface 25. Since the thrust imposed onbit 11 is downward, inlay 27 does not extend to the upper side ofbearing pin 15. Inlays 21 and 23 could be omitted if desired. Alubricant passage 29 extends through bit leg 13 and bearing pin 15 tothe upper side of bearing pin 15. A pressure compensator (not shown)supplies pressurized lubricant to passage 29.

[0019] A cutter or cone 31 mounts rotatably to bearing pin 15. Cone 31has a plurality of teeth 33 on its exterior. FIG. 1 shows teeth 33 fromall three cones 31 of bit 11 rotated into a single plane. Teeth 33 maybe hard metal inserts pressed into mating holes in the body of cone 31,as shown. Alternately, they may be steel teeth milled into the exteriorof cone 31.

[0020] Cone 31 has a central cavity 35 for rotatably mounting on bearingpin 15. Cavity 35 has a thrust shoulder 37 that is perpendicular to theaxis of cone 31 for mating with bearing pin thrust shoulder 19. A thrustwasher 39 is located between thrust shoulders 19 and 37. In thepreferred embodiment, thrust washer 39 is not fixed to either thrustshoulder 19 or 37, although it could be brazed or welded to one of theshoulders 19 or 37.

[0021] A bearing sleeve 41 is located in the cavity of cone 31 in thisembodiment to serve as part of a seal assembly. As shown in FIG. 2,bearing sleeve 41 preferably does not extend entirely 360 degrees,rather has a gap or slit on its upper side. Bearing sleeve 41 rotateswith cone 31 and slidingly engages journal bearing inlay 47 in thisembodiment. A retainer ring 43 extends around cavity 35 in engagementwith a retaining groove 45 to hold cone 31 on bearing pin 15. Anothertype of retainer uses balls. A seal assembly 47 seals the outer end ofcavity 35 to bearing pin 15.

[0022] Thrust washer 39 and bearing sleeve 41 are preferably formed of ahardened ferrous metal selected from the group consisting essentially ofiron with cobalt and alloys thereof, such as stainless steel orStellite. The material of thrust washer 39 and bearing sleeve 41 has alowest transformation temperature, which is considered to be atemperature at which the metal at least partially loses its propertiesas a hardened metal.

[0023] As illustrated in FIG. 4, a coating 49 of DLC material is appliedto at least one of the faces, preferably both, of thrust washer 39. Thethickness of coating 49 is greatly exaggerated in FIG. 4. A similar DLCcoating is optionally applied to the inner diameter of bearing sleeve41. As discussed above, DLC, or diamond-like carbon, is a form ofmeta-stable amorphous carbon or hydrocarbon compound with propertiesvery similar to those of diamond. Being amorphous, there are no grainboundaries. DLC coating is a carbon coating with a mixture of sp3 andsp2 bonds between the carbon atoms. The sp3 bond is a tetrahedral bondof carbon that forms diamond. The sp2 bond is of a type that formsgraphite. Technically, the sp3 bond means that the carbon reconfiguresone s-orbit and three p-orbits to form four identical orbits in atetrahedral configuration for bonding to the next carbon atom. The sp2bond is the hybridization of one s and two p-orbits to three sp2 orbits,which are planar. DLC has a certain percentage of both types of bonds,thus the hardness is between diamond and graphite. The proportions ofsp2 and sp3 can be varied. In addition to carbon, there is a certainamount of hydrogen in the DLC coatings. The hydrogen content comes fromthe process gas used, since normally DLC coatings are deposited by thedecomposition of a carbon and hydrogen compound. One acceptable compoundis acetylene. Also, the DLC coating may be doped with one or morealloying elements such as silicon, boron, boron nitride and one or morerefractory metallic elements, such as tantalum, titanium, tungsten,niobium or zirconium.

[0024] Thrust washer 39 and bearing sleeve 41 are first hardened,tempered and formed to the desired dimensions. Portions of thrust washer39 and bearing sleeve 41 that are not to be coated are masked off. Oneprocess to apply the DLC coating comprises depositing material from anRF (radio frequency) plasma, sustained in hydrocarbon gases, ontonegatively biased thrust washer 39 and bearing sleeve 41. In thisprocess, referred to as plasma assisted chemical vapor deposition orPACVD, thrust washer 39 and bearing sleeve 41 are heated by an electroncurrent to a temperature below their lowest transformation temperatures.Electrons from the electron current are attracted to the exposedportions of thrust washer 39 and bearing sleeve 41 from a plasma beam inthe center of the chamber. After heating, the exposed portions areetched by argon ion bombardment. For this etching process, thrust washer39 and bearing sleeve 41 are biased to a negative potential to attractargon ions from a plasma source. This process cleans the exposedsurfaces by etching.

[0025] Afterward, one or more metallic interlayers, usually chromium, isapplied from a sputter source such as a chromium target. Sputtering is asimilar process to etching, but a bias voltage is applied to thechromium target of several hundred volts. The exposed surfaces of thrustwasher 39 and bearing sleeve 41 serve as a negative electrode. Materialis removed from the chromium target surface by the impact of argon ions.This material condenses on the exposed surfaces. The metallic interlayeris used to increase adhesion and could be formed of other metals such astitanium.

[0026] After the interlayer is laid, acetylene is introduced and aplasma is ignited between the exposed surfaces of thrust washer 39 andbearing sleeve 41 and the chamber walls. The acetylene decomposes toform carbon atoms that coat the exposed surfaces on the metallicinterlayer with DLC. DLC coatings are insulating, thus the plasma forthe DLC cannot be a DC plasma, but must be an AC plasma. Typically an RFplasma is used. After coating, thrust washer 39 and bearing sleeve 41are cooled before venting the chamber. During the entire coatingprocess, the temperature will be maintained below the lowesttransformation temperature of thrust washer 39 and bearing sleeve 41.

[0027] In addition to the process described above, other processes aresuitable, including primary ion beam deposition of carbon items (IBD).Another process that may be suitable is sputter deposition of carbonwith or without bombardment by an intense flux of ions (physical vapordeposition). Another technique is based on closed field unbalancedmagnetron sputter ion plating combined with plasma assisted chemicalvapor deposition. The deposition is carried out at approximately 200° C.in a closed field unbalanced magnetron sputter ion plating system.

[0028] DLC coating 49 preferably has a thickness in the range from 1 to10 micrometers, preferably 2 to 5 micrometers and, even morespecifically, 2 to 3 micrometers. The hardness is in the range from2,000 from 5,000 Knoop, thus not as hard as diamond. Once the coatingsare formed on thrust washer 39 and thrust washer 41, these members areinstalled in cone cavity 35. Cutter or cone 31 is installed on bearingpin 15 in a conventional manner.

[0029] Laboratory tests have been conducted to demonstrate theperformance of the coating. First, thrust washer pressure-velocity testswere carried out. In one test, an uncoated stainless steel 440C thrustwasher ran against a mating surface that was coated with DLC to athickness of 2 to 3 micrometers. This pressure velocity tests showedthat the DLC coating more than doubled the load carrying capacity of thecomponent. The average load at the pressure velocity limit for thestandard was 1.6 million Newtons millimeter per second, while the DLCcoating had an average load at the pressure velocity limit of greaterthan 4.3 million Newtons millimeter per second.

[0030] Then, a wear test was carried out to demonstrate the wearresistance of the DLC coating. The results are shown in FIG. 6. Thedesignation TWI top and low refers to two thrust washers rotated againstone another, with one of the thrust washers having a DLC coating and theother being uncoated 440C stainless steel. When rotated against oneanother, the TWI thrust washers exhibited very little weight loss aftera two-hour test interrupted at 30 minute intervals (1800 seconds) tomake a weight loss measurement. The other specimens, designated TW2, hadboth top and bottom washers of 440C stainless steel without any DLCcoatings. The bottom or lower thrust washer wore significantly duringthe two-hour test.

[0031] In the embodiment of FIG. 5, bearing pin 51 does not have athrust shoulder inlay 21 or journal bearing inlay 27 as in FIG. 1.Instead, a DLC coating 53 is directly applied to the journal bearing ofbearing pin 51. A DLC coating 55 is directly applied to the thrustshoulder of bearing pin 51. DLC coatings 53, 55 are applied in the samemanner as described above and replace inlays 21 and 27. Thrust washer 39(FIG. 1) preferably has a DLC coating as previously described andslidingly engages thrust shoulder DLC coating 55. The DLC coatings 41and 55 are thus in sliding engagement with each other. Alternately, theDLC coatings could be in the cavity of the cone and on bearing pin 51,and thrust washer 39 could be conventional without DLC coatings.

[0032] As additional alternates, bearing sleeve 41 (FIG. 1) may have aDLC coating on its inner diameter as previously described that slidinglyengages DLC coating 53. As an another alternate embodiment, a DLCcoating could be applied to the outer diameter of bearing sleeve 41 andto the inner diameter of the cavity in cone 31 (FIG. 1). In thisarrangement, bearing sleeve 41 would be rotatable relative to cone 31.In such case, bearing sleeve 41 could either have DLC coatings on bothsides or no DLC coatings at all.

[0033] The invention has significant advantages. The DLC coating isapplied in a process that does not detract from the properties of thesubstrate. The DLC coating exhibits high wear resistance, with thegraphite component in the DLC coating enhancing lubrication.

[0034] While the invention has been shown in only two of its forms, itshould be apparent to those skilled in the art that it is not so limitedbut is susceptible to various changes without departing from the scopeof the invention.

1. An earth-boring bit, comprising: a bit body; a cantilevered bearingpin depending from the bit body; a cone mounted for rotation on thebearing pin; and a bearing surface between the cone and the bearing pin,the bearing surface having a DLC coating formed thereon.
 2. The bitaccording to claim 1, wherein the DLC coating has a thickness in therange from 1 to 10 micrometers.
 3. The bit according to claim 1, whereinthe DLC coating has a thickness in the range from 2 to 5 micrometers. 4.The bit according to claim 1, wherein the DLC coating has a thickness inthe range from 2 to 3 micrometers.
 5. The bit according to claim 1,wherein the DLC coating has a Knoop Scale hardness in the range from2000 to
 5000. 6. The bit according to claim 1, wherein the DLC coatingis of carbon with a mixture of sp3 and sp2 bonds between atoms of thecarbon.
 7. The bit according to claim 1, wherein the DLC coating isformed of amorphous and hydrogenated amorphous carbon.
 8. The bitaccording to claim 1, wherein the DLC coating is doped with an alloyingelement from the group consisting essentially of silicon, boron andboron nitride and a refractory metallic element from the groupconsisting essentially of tantalum, titanium, tungsten, niobium andzirconium.
 9. The bit according to claim 1, further comprising a thrustwasher located between a thrust shoulder of the bearing pin and thecone, the bearing surface containing the DLC coating being on at leastone side of the thrust washer.
 10. The bit according to claim 1, furthercomprising a sleeve located between the bearing pin and the cone, thebearing surface containing the DLC coating being on at least one side ofthe sleeve.
 11. The bit according to claim 1, further comprising athrust washer located between a thrust shoulder formed on the bearingpin and the cone, and a sleeve located between the bearing pin and thecone, the bearing surface containing the DLC coating being on at leastone side of the thrust washer and on at least one side of the sleeve.12. The bit according to claim 1, wherein the bearing surface having theDLC coating is formed on a journal surface of the bearing pin.
 13. Thebit according to claim 1, wherein the bearing surface having the DLCcoating is formed within a cavity of the cone.
 14. An earth-boring bit,comprising: a bit body; a cantilevered bearing pin depending from thebit body, the bearing pin having a thrust shoulder that is in a planeperpendicular to the axis of the bearing pin; a cone mounted forrotation on the bearing pin, the cone having a thrust shoulder facingtoward the thrust shoulder of the bearing pin; and a thrust washerlocated between and in engagement with the thrust shoulders of thebearing pin and the cone, the thrust washer having a DLC coating formedthereon on at least one side.
 15. The bit according to claim 14, whereinthe DLC coating is formed on both sides of the thrust washer.
 16. Thebit according to claim 14, wherein the thrust shoulder of the bearingpin contains an inlay of a hard wear resistant material.
 17. The bitaccording to claim 14, wherein the thrust shoulder of the bearing pinhas a DLC coating formed thereon.
 18. The bit according to claim 14,wherein the coating is of carbon with a mixture of sp3 and sp2 bondsbetween atoms of the carbon.
 19. The bit according to claim 14, whereinthe coating is formed of amorphous and hydrogenated amorphous carbon.20. The bit according to claim 14, wherein the DLC coating is doped withan alloying element from the group consisting essentially of silicon,boron and boron nitride and a refractory metallic element from the groupconsisting essentially of tantalum, titanium, tungsten, niobium andzirconium.
 21. An earth-boring bit, comprising: a bit body; acantilevered bearing pin depending from the bit body; a cone mounted forrotation on the bearing pin; and a sleeve located between the bearingpin and a cavity in the cone and having a DLC coating formed thereonthat is on at least one side.
 22. The bit according to claim 21, whereinthe DLC coating is on both sides of the sleeve.
 23. The bit according toclaim 21, wherein the bearing pin also contains a DLC coating.
 24. Thebit according to claim 21, wherein the cavity of the cone also containsa DLC coating.